Cyclobutane complexes

Many authors assume that the reaction indeed proceeds in such a way, with the specification that the quasi-cyclobutane intermediate corresponds with a complex of cyclobutane with C4-symmetry (3, 13, 2%, 46, 49, 68-72). The role of the catalyst is described by these authors in terms of the forbidden-to-allowed concept of Mango and Schachtschneider 73, 74), in which the assumption is made that the formation of the cyclobutane complex is the result of a concerted fusion of two alkenes. In the following this will be considered in more detail. 

However, because the symmetry of this complex is the same as that of the cyclobutane complex, the difference between these two intermediates... 

It is clear that a detailed mechanism for the metathesis reaction of alkenes cannot yet be given with certainty. In view of the fact that, for similar reactions which are formally cyclobutane-dialkene transformations, a nonconcerted reaction pathway has been demonstrated, a concerted fusion of two alkenes to form a cyclobutane complex and its decomposition in the same way with a change in the symmetry plane is less probable. On the basis of the information on the two other mechanisms to date, the mechanism involving a metallocyclic intermediate is more plausible than a mechanism involving carbene complexes. 

Another possibility might be a complex involving more than two alkene molecules e.g., analogous to the cyclobutane complex, a cyclohexane complex can be imagined. In the literature, evidence concerning these possibilities has not been provided so far. 

Chromium and tungsten carbene complexes containing an alkynyl or alkenyl substituent afford moderate to high yields of cyclobutene or cyclobutane complexes respectively via formal [2 - - 2]cycloaddition with... 

Liberation of ethylene from the titanium cyclobutane complex Cp 2Ti[C(=CH2)CH2CH2] generates the corresponding titana-allene species, Cp 2Ti=C=CH2 which, surprisingly, behaves differently in trapping reactions with... 

Tungsta Cyclobutan Complexes They are the reaction product of... 

Vikulov 1989b Kazansky 1991a). A small proportion (<5%) of the molybda-cyclobutane complexes yields propene by reductive elimination at 350°C. Calorimetric measurements support this scheme (Vikulov 1992a). The reaction of... 

Within the cubane synthesis the initially produced cyclobutadiene moiety (see p. 329) is only stable as an iron(O) complex (M. Avram, 1964 G.F. Emerson, 1965 M.P. Cava, 1967). When this complex is destroyed by oxidation with cerium(lV) in the presence of a dienophilic quinone derivative, the cycloaddition takes place immediately. Irradiation leads to a further cyclobutane ring closure. The cubane synthesis also exemplifies another general approach to cyclobutane derivatives. This starts with cyclopentanone or cyclohexane-dione derivatives which are brominated and treated with strong base. A Favorskii rearrangement then leads to ring contraction (J.C. Barborak, 1966). 

The dimerization of isoprene has been accompHshed by methods other than heating. Thus isoprene has been dimerized by uv radiation in the presence of photosensitizers to give a complex mixture of cyclobutane, cyclohexene, and cyclooctadiene derivatives (36,37). Sulfuric acid reportedly... 

A which is not observed in individual solutions of the two enones at the same concentrations and may thus be indicative of a complex formation. However, the ratio of isomeric cyclobutane products resulting from such photocycloadditions is generally seen to be a quite sensitive function of steric effects and of the properties of the reaction solvent, of the excited state(s) involved (in some cases two different excited triplet states of the same enone have been found to lead to different adducts) and of the substituents of the excited enone and substrate. No fully satisfactory theory has yet been put forth to draw together all the observations reported thus far. 

More detailed and theoretical explanations of the role of the catalyst, based on this scheme, have appeared (72, 74, 77-82). In order to obtain experimental evidence for this scheme, some investigators did experiments in which 1,2-dimethylcyclobutane or cyclobutane were brought into contact with an active metathesis catalyst. However, 1,2-dimethylcyclobutane was stable under conditions where propene gave a high conversion to ethene and 2-butene (63). The experiments with cyclobutane led to the same conclusion (83). From this, and from the fact that cyclobutanes are not reaction products, although this can be expected thermodynamically, it follows that cyclobutanes are not free intermediates. This prompted Lewandos and Pettit (83) to propose a tetramethylene complex as the key intermediate ... 

As an alternative to the cyclobutane mechanism there have also been proposed mechanisms involving carbene complexes2 which cannot be considered as a more detailed description of the quasi-cyclobutane intermediate, as in the case of the tetramethylene complex. H6risson and Chauvin 88) proposed the following scheme for the transalkylidenation step ... 

It has been found that certain 2 + 2 cycloadditions that do not occur thermally can be made to take place without photochemical initiation by the use of certain catalysts, usually transition metal compounds. Among the catalysts used are Lewis acids and phosphine-nickel complexes.Certain of the reverse cyclobutane ring openings can also be catalytically induced (18-38). The role of the catalyst is not certain and may be different in each case. One possibility is that the presence of the catalyst causes a forbidden reaction to become allowed, through coordination of the catalyst to the n or s bonds of the substrate. In such a case, the... 

Reactions between much stronger donors and acceptors belong to the electron tranter band. Such olefins do not form cyclobutanes but ion radical pairs or salts of olefins. refrato(dimethylamino)elhylene has an ionization potential as low as Na. The olefin with extraordinary strong electron-donating power is known not to undergo [2+2]cycloaddition reaction, but to give 1 2 complex with TCNE (transfer band in Schane 3) [23]. 

Incorporation of a flavin electron donor and a thymine dimer acceptor into DNA double strands was achieved as depicted in Scheme 5 using a complex phosphoramidite/H-phosphonate/phosphoramidite DNA synthesis protocol. For the preparation of a flavin-base, which fits well into a DNA double strand structure, riboflavin was reacted with benzaldehyde-dimethylacetale to rigidify the ribityl-chain as a part of a 1,3-dioxane substructure [49]. The benzacetal-protected flavin was finally converted into the 5 -dimethoxytri-tyl-protected-3 -H-phosphonate ready for the incorporation into DNA using machine assisted DNA synthesis (Scheme 5a). For the cyclobutane pyrimidine dimer acceptor, a formacetal-linked thymine dimer phosphoramidite was prepared, which was found to be accessible in large quantities [50]. Both the flavin base and the formacetal-linked thymidine dimer, were finally incorporated into DNA strands like 7-12 (Scheme 5c). As depicted in... 

The first heterodifunctional ligand derived from a tetradentate phosphine was made in a study concerning synthesis and characterization of a sjransj is-], 3-bis(diphenylphosphino)-2,4-bis-(diphenylphosphinothioyl)cyclobutane and its complex with PdCl2.257... 

The photosensitized dimerization of isoprene is considerably more complex than that of butadiene, yielding cyclobutanes (14)—(16) as well as four dimers of noncyclobutane types<9 11,18) ... 

An elegant example of the use of photochemistry in complex organic synthesis is the preparation the bollweevil phenomone (sex attractant). The key step in both the Zoecon Corporation synthesis and the USDA synthesis involves the formation of a cyclobutane ring by a photoaddition reaction ... 

In all the latter cases the easier dimerization reaction is connected with the particular stability of the intermediate diradical species. This is also the reason for the recently found facile dimerization of the 1-donor substituted allylidene-cyclopropane 136a (Scheme 66) [127]. Allylidenecyclopropane 136a cyclodimer-izes to the expected cyclobutane 467 in very mild thermal conditions, due to the stabilization of the intermediate 466. At higher temperature (120 °C) both 136a and 467 give a more complex mixture of products, with the cyclooctadiene dimer 468 being the prevailing one (Scheme 66) [127],... 

Though the triplet sensitized photolysis of isoprene (159) does, as noted above, produce a complex mixture of products, one of these adducts has been used in the context of complex molecule synthesis (equation 5)71. Cyclobutane 160, which was formed in ca 20% yield by the benzophenone sensitized photolysis of 159, could be easily transformed into fragrantolol, 161, an isomer of grandisol isolated from the roots of the Artemisia fragrans, by simple hydroboration/oxidation of the less hindered double bond. 

When alkenes are allowed to react with certain catalysts (mostly tungsten and molybdenum complexes), they are converted to other alkenes in a reaction in which the substituents on the alkenes formally interchange. This interconversion is called metathesis 126>. For some time its mechanism was believed to involve a cyclobutane intermediate (Eq. (16)). Although this has since been proven wrong and found that the catalytic metathesis rather proceeds via metal carbene complexes and metallo-cyclobutanes as discrete intermediates, reactions of olefins forming cyclobutanes,... 

A cyclobutane ring-opening of the photoadduct (426) by the reaction of BF3 Etfi in refluxing benzene gave isopropenylcyclohexenone (427). The reaction could be applied to more complex compounds. Thus, (428), (430), (432) and (434) were converted accordingly to (429), (431), (433) as well as (435) 145). 

The presence of a pyrrolidine unit in complex systems such as the azabicy-clo[2.1.1]hexane unit of 2,4-methanoproline (56) lends itself to synthesis pertaining to the 1,3-disubstituted cyclobutane part of the molecule. Thus two syntheses of 56 have been published, relying on the [2 + 2] photochemical synthesis of cyclobutanes. A nontrivial problem of the synthesis of these compounds is liberation of the target molecule from its protective groups (Scheme... 

Bidentate chiral water-soluble ligands such as (S,S)-2,4-bis(diphenyl-sulfonatophosphino)butane BDPPTS (Fig. 2) or (R,R) 1,2-bis(diphenylsul-fonatophosphinomethyl)cyclobutane have been prepared [25]. Their palladium complexes catalyze the synthesis of chiral acids from various viny-larenes and an ee of 43% has been reached for p-methoxystyrene with the BDPPTS ligand. Furthermore, recycling of the aqueous phase has shown that the regio- and enantioselectivity are maintained and that no palladium leaches. 

The second example is an intermolecular crystal-state reaction. Cross-conjugated 1,5-disubstituted 1,4-dien-3-ones in solution undergo both cis-trans photoisomerization and photodimerization, yielding complex mixtures of products, including die all-trans-substituted cyclobutane 2 in the case of 1,5-diphenyl-1,4-pentadien-3-one. In contrast, dienones such as 3a in whose crystals adjacent molecules lie parallel and strongly overlapped react in the solid to give 3b as the sole photoproduct. This isomerically pure tricyclic diketone results, formally, from an eight-center dimerization. It is not formed in the reaction in solution, and could be prepared by other methods only with considerable difficulty (4). 

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